The present invention generally relates to microelectromechanical systems and, more particularly to an enhanced way of moving a microstructure away from a substrate to what may be characterized as a neutral position.
There are a number of microfabrication technologies that have been utilized for making microstructures (e.g., micromechanical devices, microelectromechanical devices) by what may be characterized as micromachining, including LIGA (Lithography, Galvonoforming, Abforming), SLIGA (sacrificial LIGA), bulk micromachining, surface micromachining, micro electrodischarge machining (EDM), laser micromachining, 3-D stereolithography, and other techniques. Bulk micromachining has been utilized for making relatively simple micromechanical structures. Bulk micromachining generally entails cutting or machining a bulk substrate using an appropriate etchant (e.g., using liquid crystal-plane selective etchants; using deep reactive ion etching techniques). Another micromachining technique that allows for the formation of significantly more complex microstructures is surface micromachining. Surface micromachining generally entails depositing alternate layers of a structural material (e.g., polysilicon) and a sacrificial material (e.g., silicon dioxide) using an appropriate substrate (e.g., a silicon wafer) that functions as the foundation for the resulting microstructure(s). Various patterning operations (collectively including masking, etching, and mask removal operations) may be executed on one or more of these layers before the next layer is deposited so as to define the desired microstructure(s) from the structural material in one or more of the structural layers. After the microstructure(s) has been defined in this general manner, the various sacrificial layers are removed by exposing the microstructure(s) and the various sacrificial layers to one or more etchants. This is commonly called xe2x80x9creleasingxe2x80x9d the microstructure(s) from the substrate, typically to allow at least some degree of relative movement between at least some of the microstructures and the substrate.
Surface micromachining may be used to fabricate a mirror array that is defined by a plurality of mirror assemblies. Each mirror assembly may generally include a mirror and a mirror positioning assembly. Typically each mirror would be fabricated at a structural level in a surface micromachined system that is vertically spaced from a substrate that is used in the fabrication of the microelectromechanical system. It may be desirable to move each of these mirrors from their corresponding fabricated position (e.g., the position occupied by the mirrors in the microelectromechanical system prior to using an appropriate release etchant to remove the various layers of sacrificial material) to a position that is spaced further from the substrate prior to operating each of the various mirror assemblies. That is, it may be desirable to increase the spacing of each of the various mirrors from the substrate prior to tilting each of the various mirrors in a desired manner to provide the desired optical function.
The present invention is generally directed to microelectromechanical (MEM) systems, and more specifically, to an assembly for elevating and supporting a microstructure (e.g. a reflective microstructure/mirror) of a MEM system generally by engaging a positioning system of the MEM system with an elevator lifter. The assembly of the present invention desirably addresses a need to increase the vertical spacing between various microstructures and the substrate (generally after fabrication of the MEM system and prior to utilizing each of the various microstructures in operations of the MEM system) to allow for larger tilt angles and/or greater movement of the microstructures while preventing contact between the substrate and the microstructures. Another benefit of the present invention may be to avoid potential stiction-related problems between the mirror(s) and the substrate. Yet another benefit of increasing the spacing of the mirror(s) may be to reduce the potential for improper operation of the associated positioning assembly by pre-elevating it above a difficult-to-operate-in 0 degree horizontal position. While particularly desirable applications of the assembly may be in elevating and supporting reflective microstructures such as mirrors of an optical array/switch, the assembly of the present invention may be utilized in any appropriate microelectromechanical application for which elevation/lifting of a microstructure is desired/required.
A first aspect of the present invention relates to a microelectromechanical (MEM) system formed on a substrate. This MEM system generally includes a first microstructure (such as a mirror) disposed in vertically spaced relation to the substrate. In other words, this first microstructure is positioned above and preferably avoids direct contact with the substrate. In addition, the MEM system also includes an actuator assembly movably interconnected with the substrate. An elevator is generally pivotally interconnected with the substrate and further interconnected with the microstructure. This elevator is typically interconnected with the actuator assembly via a tether. In addition, this MEM system generally includes a first elevator lifter engageable with the elevator. By designing the first elevator lifter to engage the elevator (e.g., rather than the microstructure itself), this first aspect reduces the potential of subsequent operation of the microstructure being hindered due to inadvertent malpositioning of the microstructure between a portion of the elevator lifter and the substrate (i.e., reduces the potential for the microstructure to get caught under a portion of the elevator lifter).
Various refinements exist of the features noted in relation to the first aspect of the present invention. Further features may also be incorporated in this first aspect as well. These refinements and additional features may exist individually or in any combination. For example, this first aspect may utilize any appropriate number and/or type of actuator(s). In addition, any appropriate configuration of the tether and elevator may be utilized which provides for a pivotal interconnection of the elevator to the substrate as well as pivotal movement of the elevator with respect to the substrate. Herein, a xe2x80x9cpivotal interconnectionxe2x80x9d or the like, refers to any type of interconnection that allows a microstructure to at least generally undergo a pivoting or pivotal-like motion when exposed to an appropriate force, including without limitation any interconnection that allows a microstructure or a portion thereof to move at least generally about a certain axis. Representative pivotal interconnections include the use of a flexing or elastic deformation of a microstructure or a portion thereof, as well as the use of relative motion between two or more microstructures that are typically in interfacing relation during at least a portion of the relative movement (e.g., a hinge connection; a ball and socket connection).
Some variations of this subject first aspect may also include a displacement multiplier having an input structure and an output structure. In such variations, the actuator assembly may be interconnected with the input structure and the tether may be interconnected with the output structure. Thus, movement/actuation of one or more actuator elements (such as a moveable electrostatic comb) of the actuator assembly may be magnified to provide a motive force to move/displace the tether (and generally the structure(s) attached thereto).
The first elevator lifter of the MEM system of the first aspect may be at least initially disposed in vertically spaced relation to the elevator. Thus, in some variations, the first elevator lifter may be positioned under at least a portion of the elevator. Stated another way, the first elevator lifter may be interposed between the substrate and at least a portion of the elevator. This first elevator lifter may include one or more pre-stressed beams. The pre-stressed beam(s) of the first elevator lifter may be made up of an encased oxide (e.g., doped/undoped silicon dioxide or silicon oxide) or any other appropriate material(s). In some variations, the pre-stressed beam(s) may include upper and lower walls disposed in vertically spaced relation by a closed-perimeter sidewall that extends between and interconnects the upper and lower walls. In such variations having a sidewall and upper and lower walls, the oxide may be positioned within an enclosed space defined by the upper and lower walls and the sidewall. A thickness of the oxide positioned between the upper and lower walls of the pre-stressed beam(s) can generally be within a range of about 0.3 microns to about 6 microns, and more preferably about 3.5 microns. However, thicknesses outside the disclosed range may be appropriate.
Some variations of the subject first aspect may include a first fuse interconnected with the first elevator lifter. In such variations, the first fuse generally functions to maintain the first elevator lifter in a fixed, vertically spaced position relative to at least a portion of the elevator prior to being activated. In other words, the first fuse may function to maintain a distance of separation between the first elevator lifter and the elevator before the fuse is xe2x80x9cblownxe2x80x9d. After activation of this first fuse (i.e., after an threshold amount of voltage is applied to the fuse, causing the fuse to break), the first elevator lifter may engage at least a portion of the elevator to move and/or tilt the elevator at least generally away from the substrate. In some variations, a plurality of fuses (e.g., first and second fuses) may be utilized to hold the positioning of the first elevator lifter prior to activation of the fuses.
In some variations of the first aspect, sacrificial material may maintain the first elevator lifter in vertically spaced relation to at least a portion of the elevator prior to being released by an etchant. After release of the sacrificial material, the first elevator lifter engages at least a portion of the elevator to move the elevator at least generally away from the substrate. In other words, during an appropriate fabrication process, the first elevator lifter may be held in place (i.e., kept away from significantly biasing the elevator) by an appropriate sacrificial material, such as an oxide. When this sacrificial material is etched away during a xe2x80x9creleasexe2x80x9d step of the fabrication process, nothing remains to prevent the first elevator lifter from engaging and exerting a lifting force on at least a portion of the elevator.
First and second interconnects may extend between and interconnect the elevator with the microstructure in the case of the first aspect. These first and second interconnects may have a variety of designs/configurations. In addition, the first and second interconnects may exhibit a variety of tensile and/or elastic properties. However, these first and second interconnects preferably exhibit some compliance/flexibility. Those various features discussed above in relation to the first aspect of the present invention may be incorporated into any of the other aspects of the present invention as well, and in any appropriate manner noted herein.
A second aspect of the present invention is embodied in a method for controlling a position of a microstructure (such as a mirror) relative to a substrate. The method generally includes exerting an at least substantially upwardly-directed force directly on an elevator to move at least a portion of the elevator at least generally away from the substrate. Typically, the microstructure is interconnected with at least a portion of the elevator. Due to the exertion of the at least substantially upwardly-directed force, the microstructure is generally moved from a first position to a second position. In other words, since the microstructure is interconnected with the elevator, exertion of an upwardly-directed (i.e., oriented at least generally away from the substrate) force on the elevator tends to cause the microstructure to move from a first position to a second position.
Various refinements exist of the features noted in relation to the second aspect of the present invention. Further features may also be incorporated in this second aspect as well. These refinements and additional features may exist individually or in any combination. The exerting step may include exerting a passive force on the elevator. A xe2x80x9cpassive forcexe2x80x9d herein refers to a force that is not directly brought about by an application of voltage to the structure being affected by the force. Conversely, a xe2x80x9cpassive forcexe2x80x9d generally encompasses any force that is produced and/or brought about by a release of stored energy from a first structure. However, some variations of this second aspect may have a first structure that is made up of a material that undergoes at least one physical characteristic in response to the application of voltage (e.g., a piezo-electric material). In such variations, a xe2x80x9cpassive forcexe2x80x9d may include applying voltage to the first structure to activate the material.
In the case of the second aspect, the exertion of the at least substantially upwardly-directed force may include using a pre-stressed beam. An example of a pre-stressed beam is a beam having an inherent stress gradient (like a compressed spring) such that release of one end of the beam causes the released end to flex and reposition the beam to a relaxed position. Accordingly, this exerting step may include bending a beam that is in contact with the elevator. This bending step may include using only passive forces, such as using only energy stored in the beam.
Since the at least substantially upwardly-directed force is generally exerted directly on the elevator in the case of the second aspect, the exerting step may be generally executed on an underside of the elevator. The exerting step may include moving the elevator to a neutral position. Herein, a xe2x80x9cneutral positionxe2x80x9d refers to a location of the elevator that results when substantially no motive force is transmitted via a tether to raise/lower the elevator while and/or after the elevator has reached the second position. Some variations of the subject second aspect may include moving the elevator from the neutral position to change an orientation of the microstructure relative to the substrate. In other words, raising (i.e., moving away from the substrate) or lowering (i.e., moving toward the substrate) at least a free end of the elevator may cause tilting, movement, and/or displacement of the microstructure.
In the case of the second aspect, the microstructure (e.g., in the case where the microstructure is a mirror) may be disposed in at least substantially parallel relation to a lateral extent of the substrate when in at least one of the first and second positions. By contrast, the microstructure may exhibit an angular orientation between 0 degrees and 10 degrees in relation to a lateral extent of the substrate when in the second position. While moving the microstructure from the first position to the second position may result in changes of the angular orientation of the microstructure with respect to the substrate, the moving step may include increasing a spacing between the microstructure and the substrate. In other words, the microstructure may be spaced from the substrate by a first distance when in the first position and by a second distance, greater than the first distance, when in the second position.
In some variations of the method of the second aspect, the exerting step may also include using a lifter. This lifter may be maintained in spaced relation to the elevator prior to initiating the exerting step. One possible way of maintaining the positioning of the lifter prior to the exerting step may include anchoring a portion of the lifter with at least one fuse. In other words, the lifter can be held in a fixed position with respect to the substrate prior to carrying out the exerting step of the method. Accordingly, the exerting step may include blowing each fuse and moving the lifter at least generally away from the substrate into contact with the elevator. Those various features discussed above in relation to the second aspect of the present invention may be incorporated into any of the other aspects of the present invention as well, and in any appropriate manner noted herein.